Split or separable core current transformers



29, 1967 E. c. WENTZ ETAL 3,339,163

SPLIT OR SEPARABLE CORE CURRENT TRANSFORMERS Filed Jan. 29, 1965 INVENTORS Edward C. Wenfz and Belvin B. Ellis ATTORNEY United States Patent Office 3,339,163 Patented Aug. 29, 1967 3,339,163 SPLIT R SEPARABLE CORE CURRENT TRANSFORMERS Edward C. Wentz, Sharpsville, and Belvin B. Ellis, Pulaski, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Jan. 29, 1965, Ser. No. 428,908 6 Claims. (Cl. 336-210) ABSTRACT OF THE DISCLOSURE A current transformer having an openable two part magnetic core, with a winding disposed on each part. The two parts of the magnetic core when joined form two oppositely disposed stepped-lap joints, formed by a plurality of radially superposed half turns of laminations in each core part. The step-lap joints interlock to maintain the two core parts in assembled relation, and they are separable by axially moving one core part with respect to the other.

In current transformers of the clamp-on type, wherein the core of the transformer is split to permit the transformer to be clamped onto a current carrying conductor, it has been the practice in the past to use butt joints of the type known as the C-core, or joints of the butt lap type for joining the two parts of the magnetic core after the core has been placed around the current carrying conductor. These transformers have performed poorly because of the high and variable reluctance in the butt or the butt lap joints used to join the two halves of the core to complete the magnetic circuit around the current carrying conductor. This high reluctance in the joints causes a high and variable error in the meter used with these conventional split core type current transformers.

It is an object of this invention to provide an improved current transformer using a split, or two-part, core wherein the two parts of the core are joined with low and constant reluctance joints.

It is a further object of this invention to provide a current transformer having a split, or two-part core wherein the transformer permits accurate meter readings.

It is a further object of this invention to provide an improved current transformer having a split, or two-part, core wherein the two parts of the core are joined to complete the magnetic circuit by low and constant reluctance, step lap joints.

Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.

For a fuller understanding of the nature and objects of this invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

FIGURE 1 is a side view of a two-part core used in this invention, showing the step lap joints;

FIG. 2 is a side view showing a current transformer as provided by this invention assembled around a current carrying conductor;

FIG. 3 is a top view of the transformer illustrated in FIG. 2; and

FIG. 4 is a perspective view, illustrating the manner in which a transformer of the type illustrated by FIG. 2 is assembled about a current carrying conductor.

Through the various figures of the drawing like reference characters refer to like elements.

Referring to the drawing in detail, magnetic core such as provided by this invention is shown in FIG. 1. The magnetic core 10 of FIG. 1 is provided by winding up a plurality of turns of oriented magnetic sheet steel until the desired build-up of the core 10 is obtained. After the core 10 is wound to the desired build-up, the core 10 is then annealed to relieve stresses locked into the turns of magnetic sheet steel during the winding operation. Then the magnetic core 10 is cut completely across to provide two cuts in the core 10 at approximately 180 apart. Then the half turns of magnetic steel laminations provided by the two cuts are divided into groups, such as indicated by the reference characters 14, 16, 18 and 20. After the half turns of magnetic steel have been divided into groups then each half turn in each group is shifted with respect to each adjacent half turn, as indicated at 21, 22, 24, 26, 28, 30 and 32, so that the cut ends of the half turns in each group forms a step lap joint pattern in the group of half turns. It has been found from experiment that an overlap of approximately 6 to 1, that is with the portion of the lamination overlapping the cut end of the next adjacent lamination being approximately six times the thickness of the lamination, provides a joint having excellent reluctance characteristics. The laminations used to wind the core 10 of FIG. 1 are approximately .012 inch thick. After the half turns in each of the groups of laminations has been shifted, then the groups 14, 16, 18 and 20 are placed concentrically as illustrated in FIG. 1 so that the step lap joints of the combined group of half turns provides a complete step lap joint, which is repetitive in each group of half turns, completely across the core 10. It is emphasized that the cut ends of the half turn laminations in the core 10 are substantially aligned with each other. After the half turns of magnetic sheet steel have been shifted to provide step lap pattern joints at the two cuts 41 and 43 in the core 10, as illustrated in FIG. 1, then portions of the edges of the laminations are coated with a thermosetting resin composition, such :as an epoxy resin, as indicated by the reference character 40. The resin 40 is then cured. After the resin 40 is completely cured it serves to maintain the half turn laminations of the core 10 in the position to which they have been orignally shifted. This permits assembling and disassembling of the core 10 around a current carrying conductor in which the current is to be measured without upsetting or distorting the originally arranged position of the laminations in the core 10. It

is seen that the core 10 has two cuts 41 and 43, approximately apart, completely across the laminations or turns. The joints at each of the cuts 41 and 43 in the core 10 is made up of a plurality of step lap joints provided by shifting the half turns or laminations as described above. It is observed from FIG. 1 that the joint at each of the cuts 41 and 43 is identically the same. This facilitates assembling and disassembling of the core about a current carrying conductor.

Referring to FIG. 2, this figure illustrates a transformer utilizing a core 10, as illustrated in FIG. 1. The transformer is assembled about a conductor 42 in which the current is to be measured. The conductor 42 functions as the primary of the transformer. The transformer comprises a magnetic core 10 made up of two halves 44 and 46 having step lap joints therein which are provided in the manner described for providing the step-lap joint in the core of FIG. 1. However, it is to be noted that the core of FIG. 2 is comprised of only two groups and that the step lap joints in the two groups are repetitive or aligned so as to provide a complete step-lap joint completely across the core at two places 41 and 43, substantially 180 apart, to permit assembly or disassembly of the core 10 around the current carrying conductor 42. The core section 44 is provided with a secondary winding 48 and the core section 46 is provided with a secondary winding section 50. The secondary windings 48 and 50 may be wound directly onto the core sections 44 and 46, with the core sections 44 and 46 providing a bobbin for winding the windings 48 and 50, or the windings 48 and 50 may be preformed and then threaded onto the core sections 44 and 46. Like in the core described in FIGURE 1 the edges of the core sections 44 and 46 of FIG. 2 are also coated with a thermosetting resin to prevent the laminations from shifting from their originally arranged position. After the core sections 44 and 46 have been assembled around the current carrying conductor 42 the windings 48 and 50 are connected in series by means of a clamp arrangement 52. Terminals 54 and 56 are provided for connecting a meter or other instrument to the coils 48 and 50.

If it is desired that the transformer be permanently mounted about the current carrying conductor 42 a clamp is provided to clamp each of the joints 41 and 43 of the core sections 44 and 46 to eliminate any possibility of the two joints 41 and 43 in the core sections 44 and 46 from becoming loose. This clamping arrangement includes a bar member 58 located on the inside, or in the window, of the core and another bar member 60 located on the outside of the core 10. The two bar members 58 and 60 are clamped together, and against the joints 41 and 43 in the core 10 by means of two screws 62 and 64. It is observed that the two screws 62 and 64 are beyond the edges of the core 10 and therefore do not require providing holes in the core laminations to accommodate the screws 62 and 64. This clamping arrangement is further illustrated by FIG. 3 which is a top view of a transformer arrangement shown in FIG. 2.

FIG. 4 illustrates a slightly different embodiment of the transformer of FIG. 2. In the illustration of FIG. 4, the core 10 is provided from two groups of laminations which have been cut and shifted to provide two core sections 44 and 46, as illustrated in FIG. 2, and the half turns or laminations have been shifted the same as in FIG. 2 to provide two step-lap joints 41 and 43 in the core 10, with the joints located substantially 180 apart around the core, with the laminations shifted so that the overlap of adjacent laminations is not more than six times the thickness of an adjacent lamination, and with the cut ends of the half turn laminations substantially aligned with each other. In the embodiment illustrated in FIG. 4 the secondary coils 48 and 50 are preformed and a resin coating 66 is cast around the coils. Then the coils are threaded onto the core sections 46 and 48.

FIG. 4 also illustrates the manner in which the core sections 44 and 46 are assembled about the current carrying conductor 42. To assemble the core sections 44 and 46 about the current carrying conductor 42 the following procedure is followed; the core section 44 is first positioned around the conductor 42, then the core section 46 is positioned around the conductor 42 and axially displaced from the core section 44, then the core section 46 is moved axially in the direction of the arrow A until the step-laps in the half turns or laminations at the cuts in the section 46 mates with the step-lap joints in the section 44 to provide a tight fitting low reluctance joint at the two cuts 41 and 43 in the core sections 44 and 46. After the two core sections 44 and 46 have been fitted together in this manner the core sections will remain so assembled until one other of the sections 44 or 46 is shifted axially relative to the other section. It is not necessary to clamp the core sections 44 and 46, since the interlocking effect of the step lap joints in the half turns or laminations of the core sections 44 and 46 will hold the core sections 44 and 46 in assembled relationship about the conductor 42.

It has been found by experiment that the core as provided by this invention; wherein, a plurality of laminations are wound to the desired build-up and the laminations are out completely across at two points, substantially 180 apart, the laminations divided into groups, and each lamination in each of the groups is shifted to provide a step lap joint for joining the laminations in each of the core halves, that the joints so provided in the core halves have very low reluctance, in fact some of the cores appear from a loss standpoint to be almost equal to non-cut cores. This construction provides a very low reluctance and low loss core which permits the construction of a very accurate current transformer. More important the reluctance does not vary with elapse of turns nor will it vary if the core is subsequently assembled and disassembled any number of times.

An advantage of the transformer provided by this invention is, that since each half of the cut core is provided with step lap joints at the ends of the laminations, when the core is fitted around the current carrying conductor that the core may be assembled around the current carrying conductor and removed and reassembled around the conductor many times without increasing the reluctance of the joint between the two core halves.

Another important advantage of the current transformer constructed according to the teachings of this invention is that the step lap joints between the two core halves have not only very low reluctance but a stable constant low value of reluctance; and, consequently, a much more accurate meter reading is provided by the transformer than by the prior art current transformers having a divided core. In the prior art devices having a divided core the reluctance at the joints was very high and it varied with assembly and disassembly and this resulted in a distorted meter reading.

Since numerous changes may be made in the abovedescribed transformer and different embodiments of the transformer may be made without departing from the spirit and scope thereof, it is in tended that all of the matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A current transformer comprising a two-part, openable magnetic core, said core being adapted to be placed around a conductor carrying current to be measured, said core comprising a plurality of radially superposed turns of oriented sheet steel, said turns being cut at two places to provide two core parts, each having a like number of similarly radially positioned half turns of laminations, the. cut ends of the two half turns of each turn substantially abutting each other in each turn, the cut ends of each half turn in each turn being shifted with respect to the cut ends of each half turn in each adjacent turn so that the cut ends of each said half turn overlap the cut ends of the adjacent half turn to provide two interlocking step-lap joints in said core at the cut ends of said half turns, the interlocking step-lap joints in said magnetic core maintaining its two parts in assembled relation, and allowing said magnetic core to be opened by axial movement of one part of the magnetic core relative to the other, and windings around each part of said core provided by said half turns.

2. A current transformer comprising a two-part, openable magnetic core, said core being adapted to be placed around a conductor carrying current to be measured, said core comprising a plurality of radially superposed turns of oriented sheet steel, said turns being cut at two places to provide two core parts, each having a like number of similarly radially positioned half turns of laminations, said half turns of laminations being divided into a plurality of groups of laminations, the cut ends of the two half turns of each turn in each of said groups substantially abutting each other, the cut ends of each half turn in each turn of each group being shifted with respect to each adjacent half turn in each group so that the cut ends of each half turn overlap the cut ends of the adjacent half turn in each turn to provide interlocking step-lap joints at the cut ends of each of said groups, said groups being arranged concentrically with respect to each other so that the step-lap joints in said groups provide two step-lap joints completely across said core,

the interlocking step-lap joints in said magnetic core maintaining its two parts in assembled relation, and

allowing said magnetic core to be opened by axial movement of one part of the magnetic core relative to the other, and windings on said core.

3. A current transformer comprising a two-part, openable magnetic core, said core being adapted to be placed around a conductor carrying current to be measured, said core comprising a plurality of radially superposed turns of oriented sheet steel, said turns being cut in two places to provide two core parts, each having a like number of similarly radially positioned half turns of laminations, the cut ends of the two half turns in each turn substantially abutting each other in said core, the ends of each half turn in each turn being shifted with respect to the ends of each adjacent half turn in each adjacent turn so that the cut ends of each half turn overlap the cut ends of the adjacent half turn to provide interlocking step-lap joints at the cut ends of the half turns in said core, resin applied to the edges of said turns to maintain said out ends of said half turns in the shifted position, the interlocking step-lap joints in said magnetic core maintaining its two parts in assembled relation, and allowing said magnetic core to be opened by axial movement of one part of the magnetic core relative to the other, and a winding on each part of said core.

4. A current transformer comprising a two-part magnetic core, said core being adapted to be placed around a conductor carrying current to be measured, said core comprising a plurality of radially superposed turns of oriented sheet steel, said turns being cut at two places to provide two core parts, each having a like number of similarly radially positioned half turns of laminations, the cut ends of the two half turns in each turn substantially abutting each other, the ends of each half turn in each turn being shifted with respect to the ends of each adjacent half turn in each adjacent turn so that the cut ends of each half turn overlap the cut ends of the adjacent hal-f turn to provide two interlocking steplap joints across said core, the interlocking step-lap joints in said magnetic core facilitating the assembly of the two parts of the core by axial movement of one part of the core relative to the other, clamping means for maintaining said step-lap joints in the assembled position, and a winding on each part of said core.

5. A current transformer comprising a two-part magnetic core, said core being adapted to be placed around a conductor carrying current to be measured, said core comprising a plurality of radially superposed turns of oriented sheet steel, said turns being cut at two places to provide two core parts, each having a like number of similarly radially positioned half turns of laminations, the cut ends of the two half turns in each turn substantially abutting each other, the end of each half turn being shifted with respect to the end of each adjacent half turn so that the cut end of each half turn overlaps the cut end of the adjacent half turn to provide two interlocking step-lap joints in said core, the interlocking step-lap joints in said magnetic core facilitating the assembly of the two parts of the core by axial movement of one part of the core relative to the other, a bar positioned in the window of said core adjacent each of said step-lap joints and a bar positioned on the outside of said core adjacent each of said step-lap joints and means clamping said bars together to maintain said step-lap joints in assembled relationship, and a winding on each part of said core.

6. A current transformer comprising a two-part, openable magnetic core, said core being adapted to be placed around a conductor carrying current to be measured, said core comprising a plurality of radially superposed turns of oriented sheet steel, said turns being cut at two places to provide two core parts, each having a like number of similarly radially positioned half turns of laminations, the cut ends of the two half turns in each turn substantially abutting each other, the ends of each half turn being shifted with respect to the ends of each adjacent half turn so that the cut ends of each half turn overlap the cut ends of the adjacent half turns to provide two interlocking step-lap joints in said core, the' interlocking step-lap joints in said magnetic core maintaining its two parts in assembled relation, and allowing said magnetic core to be opened by axial movement of one part of the magnetic core relative to the other, said overlap of said half turns being not greater than six times the thickness of a lamination, and windings on each part of said core.

References Cited UNITED STATES PATENTS 750,525 1/1904 Everest 336- X 2,456,459 12/ 1948 Somerville 336-217 X 2,579,560 12/1951 Ford 336-219 X 2,931,993 4/ 1960 Dornbush 336--217 2,973,494 2/1961 Ellis 336-217 3,001,163 9/1961 Pfuntner et al. 336217 LEWIS H. MYERS, Primary Examiner. T. J. KOZMA, Assistant Examiner. 

1. A CURRENT TRANSFORMER COMPRISING A TWO-PART OPENABLE MAGNETIC CORE, SAID CORE BEING ADAPTED TO BE PLACED AROUND A CONDUCTOR CARRYING CURRENT TO BE MEASURED SAID CORE COMPRISING A PLURALITY OF RADIALLY SUPERPOSED TURNS OF ORIENTED SHEET STEEL, SAID TURNS BEING CUT AT TWO PLACES TO PROVIDE TWO CORE PARTS, EACH HAVING A LIKE NUMBER OF SIMILARLY RADIALLY POSITIONED HALF TURNS OF LAMINATIONS, THE CUT ENDS OF THE TWO HALF TURNS OF EACH TURN SUBSTANTIALLY ABUTTING EACH OTHER IN EACH TURN, THE CUT ENDS OF EACH HALF TURN IN EACH TURN BEING SHIFTED WITH RESPECT TO THE CUT ENDS OF EACH HALF TURN IN EACH ADJACENT TURN SO THAT THE CUT ENDS OF EACH SAID HALF TURN OVERLAP THE CUT ENDS OF THE ADJACENT HALF TURN TO PROVIDE TWO INTERLOCKING STEP-LAP JOINTS IN SAID CORE AT THE CUT ENDS OF SAID HALF TURNS, THE INTERLOCKING STEP-LAP JOINTS IN SAID MAGNETIC CORE MAINTAINING ITS TWO PARTS IN ASSEMBLED RELATION, AND ALLOWING SAID MAGNETIC CORE TO BE OPENED BY AXIAL MOVEMENT OF ONE PART OF THE MAGNETIC CORE RELATIVE TO THE OTHER, AND WINDINGS AROUND EACH PART OF SAID CORE PROVIDED BY SAID HALF TURNS. 